If you think you won’t really want to change gear when riding your bike, single speed may be suitable. It is more efficient than multi-speed in a similar gear, meaning the bike won’t require as much energy to keep it moving. Since single speed wheels do not need to be dished, they can be built symmetrical which means they are stronger than a derailleur wheel with the same number of spokes. The simplicity of the single speed wheel means that removing and refitting it (if there’s a puncture) is simple.
The disadvantage of single speed is that you’re stuck with that gear, no matter whether you’re carrying luggage on the bike, you’re riding into a headwind or it’s the end of the day and you’re tired.
Some people fit a flip-flop hub with a larger freewheel for these circumstances. This requires some method of adjusting the chain tension. On a frame with horizontal dropouts, this can be done by varying the position of the rear wheel in the dropouts (move the wheel backwards for more tension, move it forward for less). On a frame with vertical dropouts, you can take up the slack using a chain tensioner. Tensioners are specially made for converting bikes with vertical dropouts to single speed. They decrease efficiency slightly, but not as much as a derailleur.
Single speed is a very good choice for new cyclists, who have enough to think about besides changing gear, as long as the gear chosen suits the rider and the terrain. Some cyclists ride a multi-speed bike without changing gear anyway. If this is you, with single speed you won’t miss the choice of gears and you will benefit from a more robust, more efficient bike that needs less maintenance.
Determining which single speed gear to use for your journey
To pick the correct single speed gear for your journey, ride your regular journey on a derailleur geared bike with your usual amount of cycle luggage and note which sprocket you use to climb the steepest hill on that route. Count the teeth on the rear sprocket. That’s the size of single speed sprocket to choose, if you’ll be using the same size chainring when you go single speed. In practice, you may be able to use a gear 5-10% taller than that, when you don’t have a derailleur working against you, if you don’t plan on carrying any more luggage than this.
There’s detailed information on single speed at www.sheldonbrown.com